Various bacteria are present in the oral cavity, and those that cause caries are called cariogenic bacteria. For example, it is thought that a high cariogenic bacterial count in saliva indicates that the caries risk, i.e. a risk that indicates to what extent the conditions of the oral cavity are likely to cause caries, is high. Known examples of the method for detecting cariogenic bacteria include detection methods using resazurin, which is an oxidation-reduction indicator (Patent Documents 1 and 2), and detection methods utilizing chemiluminescence based on a reducing power such as NADH (Patent Document 3).
Resazurin is usually present as resazurin, which is an oxidized blue pigment (maximum absorption wavelength, 605 nm), but it is reduced by NADH or NADPH (these may be hereinafter collectively referred to as “NAD(P)H”) produced as a result of bacterial metabolism to be converted into resorufin, which is a reddish purple pigment (maximum absorption wavelength, 573 nm). Resorufin is further reduced by NAD(P)H to be converted into hydroresazurin, which is colorless. Therefore, a color change of resazurin allows measurement of the number of bacteria, mainly gram-positive bacteria including cariogenic bacteria, in a test substance. Known examples of measurement systems for the cariogenic bacterial count utilizing resazurin include RD test “Showa” (trade name; Showa Yakuhin Kako Co., Ltd.) and CAT21 Fast (trade name; Willdent Co., Ltd.).
Electron carriers such as 1-methoxy-5-methylphenazinium methyl sulfate (1-methoxy PMS), phenazinium methyl sulfate (PMS), and phenazine ethosulfate (PES) are used, for example, reaction systems using a tetrazolium salt as a color reagent, as a mediator for oxidation-reduction reaction of the tetrazolium salt. Although it is said that PMS can be used as a mediator in reaction systems using resazurin as an oxidation-reduction indicator (Patent Document 4), diaphorase is said to be more preferred than PMS as the mediator (Patent Document 4).
However, use of a salt such as sodium chloride or potassium chloride in such reaction systems using an oxidation-reduction indicator and a mediator has not been known.